The DOB is important. It isn't printed on the BP or available to a TSA Officer at the checkpoint, but it is used by TSA's advance Secure Flight program to check for matches to no fly lists and all manner of other databases.

Falsifying the PNR is a big mistake. While it may all go unnoticed, it can also cause the SF system to flag the passenger and that can cause delays. In the end, the passenger won't be denied boarding, but he may receive extra screening and other unnecessary stuff.

Simply call or email the speaker and let them know that you need this information in order to book their ticket and that you need it ASAP.

For others, this is always a bit touchy, but people who are used to traveling for others in one-off situations, e.g. speakers at events, understand why you need the information. You can make it clear that it's for booking purposes only and that you won't retain the information.